SPI.send4bit can't maintain clock at the same time as USB

[gfwilliams]

Causes glitches like are seen here:

http://www.youtube.com/watch?v=RBHP4xDCRk4

I guess ideally SPI would be driven via interrupts, so this fix could form part of a larger bit of work

[gfwilliams]

There doesn't seem to be enough leeway to handle USB as well as doing this (there's an spi_irq branch which attempts to handle SPI via IRQ, and it really struggles at this datarate), so I just disabled this on this commit: 514e32c